The United States Government as well as many others have been developing triggered barriers for suppressing incipient as well as fully developed gas and dust explosions in underground coal mines or other industries where combustible gases or dust accumulate. In order for such barriers to be effective, it is important that incipient explosions be detected early and the barrier unit activated so as to contain the explosions. Presently, all triggered barriers consist of three essential elements: a flame or explosion sensor, some type of a flame extinguisher dispensing system, and an effective extinguishing agent. Most triggered barriers have an optical or mechanical device to sense the developing explosion, and an electronic signal processing package to process the sensed signal and then activate the dispenser which rapidly dispenses the extinguishing agent. The extinguishant, which can be gas, liquid, powder, or some combination is discharged by some form of stored energy, such as explosive or gas pressure.
All of the known rapidly activated triggering systems require an external power source to activate the triggered barrier. In most coal mines, it is difficult to provide power to those locations where explosion protection is needed.
As appreciated by those of ordinary skill in the art, coal dust explosions are most often initiated by an electrical or frictional ignition of a pocket or layer of a flammable methane-air mixture that causes a local aerodynamic disturbance with sufficient violence to scour up, disperse, and ignite coal dust lying on the mine surfaces. If conditions are right, the resulting flame grows into a self-sustaining coal dust explosion, and can propagate for long distances in a mine. Triggered barriers are thus being used worldwide in potentially hazardous regions of mines.
A typical installation includes a sensor located in the proximity of the suspected explosion source, and an extinguishant dispersal unit located sufficiently far from the sensor to provide time for the extinguishing agent to be discharged prior to flame arrival. It has been found that maximum effectiveness of the extinguishant is attained when it is ejected rapidly and early enough to blanket the entire mine entry cross section prior to flame arrival. If the extinguishant is dumped prematurely, the extinguishant is driven downstream so that its concentration is diluted by the explosion-induced wind before being overtaken by the flame. When the trigger signal is late, the extinguishant is dumped behind the flame where it has minimal effect.
One prior art system for sensing flames or explosions and for activating triggering barriers in mines includes devices which sense directly the temperature rise of an explosion, such as a thermocouple. These devices are limited to sensing the flame at a single point in space, and may trigger late when the flame front does not fill the entire entry cross section. In order to combat poor response time, extremely fine thermocouple wires (5 micrometers) are used. However, such fine thermocouple wires are fragile and easily damaged by shocks or impacts. In addition, such thermocouples may be falsely triggered by incidental flames or heat sources from welding torches or the like.
Another current system for sensing flames or explosions and for activating triggering barriers in mines includes devices that detect ultraviolet, visible, infrared, or black body radiation emitted by the flame. Unfortunately, such devices can be tripped by one or more false signals generated by miners' cap lamps, illumination lights on vehicles or equipment, sparks, arcs and hot surfaces.
Yet another current system includes devices which respond to dynamic wind forces preceding the flame, such as wind vanes. Unfortunately, such devices are slow to respond, sensitive to shocks and impacts, and can respond falsely to a roof fall or blasting. Such systems can also fire prematurely during a dust explosion since there is little or no correlation between wind velocity and flame location.
Still another current system includes devices which fire in response to a static pressure rise. Such devices have some of the same faults and disadvantages as devices which respond to wind, and thus they also respond falsely and prematurely.
Another suppression system is disclosed in U.S. Pat. No. 4,173,140 (Leibman et al). This system uses a sensor-trigger device having dual infrared flame sensors in combination with a pressure-arming unit. This combination is intended to prevent false and premature triggering. The pressure sensing element switches on battery power to fire a detonator when a static pressure rises above 0.04 atm (0.5 psi), and each flame sensor views a separate narrow vertical field and must operate in coincidence to turn on a firing relay. Each of the infrared flame sensors sights across the mine entry at a horizontal angle of about 25.degree. apart and through a vertically oriented slit. As each sensor must also detect radiation concurrently to energize firing, this minimizes the possibility of false triggering by small radiation sources such as miners' cap lamps when located at some distance from the sensors.